The disclosed invention is generally in the field of assays for detection of analytes, and specifically in the field of nucleic acid amplification and detection.
A number of methods have been developed which permit the implementation of extremely sensitive diagnostic assays based on nucleic acid detection. Most of these methods employ exponential amplification of targets or probes. These include the polymerase chain reaction (PCR), ligase chain reaction (LCR), self-sustained sequence replication (3SR), nucleic acid sequence based amplification (NASBA), strand displacement amplification (SDA), and amplification with Qxcex2 replicase (Birkenmeyer and Mushahwar, J. Virological Methods, 35:117-126 (1991); Landegren, Trends Genetics, 9:199-202 (1993)).
While all of these methods offer good sensitivity, with a practical limit of detection of about 100 target molecules, all of them suffer from relatively low precision in quantitative measurements. This lack of precision manifests itself most dramatically when the diagnostic assay is implemented in multiplex format, that is, in a format designed for the simultaneous detection of several different target sequences.
In practical diagnostic applications it is desirable to assay for many targets simultaneously. Such multiplex assays are typically used to detect five or more targets. It is also desirable to obtain accurate quantitative data for the targets in these assays. For example, it has been demonstrated that viremia can be correlated with disease status for viruses such as HIV-1 and hepatitis C (Lefrere et al., Br. J. Haematol., 82(2):467-471 (1992), Gunji et al., Int. J. Cancer, 52(5):726-730 (1992). Hagiwara et al., Hepatology, 17(4):545-550 (1993), Lu et al., J. Infect. Dis., 168(5):1165-8116 (1993), Piatak et al., Science, 259(5102):1749-1754 (1993), Gupta et al., Ninth International Conference on AIDS/Fourth STD World Congress, Jun. 7-11, 1993, Berlin, Germany, Saksela et al., Proc. Natl. Acad. Sci. USA, 91(3):1104-1108 (1994)). A method for accurately quantitating viral load would be useful.
In a multiplex assay, it is especially desirable that quantitative measurements of different targets accurately reflect the true ratio of the target sequences. However, the data obtained using multiplexed, exponential nucleic acid amplification methods is at best semi-quantitative. A number of factors are involved:
1. When a multiplex assay involves different priming events for different target sequences, the relative efficiency of these events may vary for different targets. This is due to the stability and structural differences between the various primers used.
2. If the rates of product strand renaturation differ for different targets, the extent of competition with priming events will not be the same for all targets.
3. For reactions involving multiple ligation events, such as LCR, there may be small but significant differences in the relative efficiency of ligation events for each target sequence. Since the ligation events are repeated many times, this effect is magnified.
4. For reactions involving reverse transcription (3SR, NASBA) or klenow strand displacement (SDA), the extent of polymerization processivity may differ among different target sequences.
5. For assays involving different replicatable RNA probes, the replication efficiency of each probe is usually not the same, and hence the probes compete unequally in replication reactions catalyzed by Qxcex2 replicase.
6. A relatively small difference in yield in one cycle of amplification results in a large difference in amplification yield after several cycles. For example, in a PCR reaction with 25 amplification cycles and a 10% difference in yield per cycle, that is, 2-fold versus 1.8-fold amplification per cycle, the yield would be 2.025=33,554,000 versus 1.825=2,408,800. The difference in overall yield after 25 cycles is 14-fold After 30 cycles of amplification, the yield difference would be more than 20-fold.
Accordingly, there is a need for amplification methods that are less likely to produce variable and possibly erroneous signal yields in multiplex assays.
It is therefore an object of the disclosed invention to provide a method of amplifying diagnostic nucleic acids with amplification yields proportional to the amount of a target sequence in a sample.
It is another object of the disclosed invention to provide a method of detecting specific target nucleic acid sequences present in a sample where detection efficiency is not dependent on the structure of the target sequences.
It is another object of the disclosed invention to provide a method of determining the amount of specific target nucleic acid sequences present in a sample where the signal level measured is proportional to the amount of a target sequence in a sample and where the ratio of signal levels measured for different target sequences substantially matches the ratio of the amount of the different target sequences present in the sample.
It is another object of the disclosed invention to provide a method of detecting and determining the amount of multiple specific target nucleic acid sequences in a single sample where the ratio of signal levels measured for different target nucleic acid sequences substantially matches the ratio of the amount of the different target nucleic acid sequences present in the sample.
It is another object of the disclosed invention to provide a method of detecting the presence of single copies of target nucleic acid sequences in situ.
It is another object of the disclosed invention to provide a method of detecting the presence of target nucleic acid sequences representing individual alleles of a target genetic element.
Disclosed are compositions and a method for amplifying nucleic acid sequences based on the presence of a specific target sequence or analyte. The method is useful for detecting specific nucleic acids or analytes in a sample with high specificity and sensitivity. The method also has an inherently low level of background signal. Preferred embodiments of the method consist of a DNA ligation operation, an amplification operation, and, optionally, a detection operation. The DNA ligation operation circularizes a specially designed nucleic acid probe molecule. This step is dependent on hybridization of the probe to a target sequence and forms circular probe molecules in proportion to the amount of target sequence present in a sample. The amplification operation is rolling circle replication of the circularized probe. A single round of amplification using rolling circle replication results in a large amplification of the circularized probe sequences, orders of magnitude greater than a single cycle of PCR replication and other amplification techniques in which each cycle is limited to a doubling of the number of copies of a target sequence Rolling circle amplification can also be performed independently of a ligation operation. By coupling a nucleic acid tag to a specific binding molecule, such as an antibody, amplification of the nucleic acid tag can be used to detect analytes in a sample. Optionally, an additional amplification operation can be performed on the DNA produced by rolling circle replication.
Following amplification, the amplified probe sequences can be detected and quantified using any of the conventional detection systems for nucleic acids such as detection of fluorescent labels, enzyme-linked detection systems, antibody-mediated label detection, and detection of radioactive labels. Since the amplified product is directly proportional to the amount of target sequence present in a sample, quantitative measurements reliably represent the amount of a target sequence in a sample. Major advantages of this method are that the ligation operation can be manipulated to obtain allelic discrimination, the amplification operation is isothermal, and signals are strictly quantitative because the amplification reaction is linear and is catalyzed by a highly processive enzyme. In multiplex assays, the primer oligonucleotide used for DNA replication can be the same for all probes.
The disclosed method has two features that provide simple, quantitative, and consistent amplification and detection of a target nucleic acid sequence. First, target sequences are amplified via a small diagnostic probe with an arbitrary primer binding sequence. This allows consistency in the priming and replication reactions, even between probes having very different target sequences. Second, amplification takes place not in cycles, but in a continuous, isothermal replication: rolling circle replication. This makes amplification less complicated and much more consistent in output.